8 Subtropical Vegetable-Gardening 



Nitrogen (or ammonia) is the most expensive fertilizer, 

 the one which is most easily lost by leaching, and also the 

 one an excess or deficiency of which may have the most 

 prejudicial effect on a crop. Hence, it behooves the user 

 to be especially careful with his nitrogenous fertilizers. 

 The cheapest way to secure nitrogen (or ammonia) is to 

 get it free from the air by growing velvet beans or other 

 legumes. 



SOURCES OF PHOSPHORIC ACID 



A source of phosphoric acid of great commercial impor- 

 tance is the phosphate rock of Florida and South Carolina. 

 This rock contains a varying quantity of phosphoric acid, 

 therefore it is necessary to make an analysis of it before 

 one can know its value. The natural rock becomes soluble 

 only slowly in the soil, so it is customary to grind it and 

 treat with sulfuric acid to make the phosphoric acid readily 

 soluble and available to plants. In this condition it is 

 known as "acid phosphate," or "superphosphate." It 

 has been found of advantage, in some soils, to use the 

 natural rock, finely ground; this is called "floats." 



In the manufacture of steel, the phosphorus of the 

 molten iron is oxidized and absorbed by lime. The re- 

 sulting slag, called "Thomas slag," contains phosphoric 

 acid and much lime. It is not acid but basic, and so is 

 suitable for acid soils, when finely ground. 



Another source of commercial importance was phos- 

 phate from the bones of various animals. Before the dis- 

 covery of phosphate rock, this was a very important 

 source ; and while the price of ground bone for fertilizer 

 has not declined much as a result of the discovery of rock 



